Image processing apparatus, image processing method, computer program, and storage medium

ABSTRACT

The present invention provides an image processing apparatus, and an image processing method, which can reproduce an edge portion with a high resolution in a high resolution raster image, and suppress image degradation such as a jaggy, while suppressing processing cost. In the present invention, an image processing part divides image data of raster data into blocks of M×N pixels (M, N are integers of 1 or more and also at least one of M and N is an integer of 2 or more). Then, the part performs color rounding processing in the block and counts the number of colors in the block. Subsequently, when the number of colors existing in the block is two, the part acquires shape information about arrangement of the two colors. Then, the part stores the shape information and color information for a first color and color information for a second color.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus, imageprocessing method, computer program, and storage medium. The presentinvention relates more specifically to the image processing apparatus,image processing method, computer program, and storage medium, which cansave memory and suppress an image processing load while maintainingdetailed information of a high resolution image.

2. Description of the Related Art

An image printing apparatus generates a multiple value bitmap imagewhich is rasterized in the apparatus. Usually this bitmap image isgenerated according to a resolution (600 dpi, 1200 dpi, or the like) ofan output apparatus (image printing apparatus).

In fine characters or line drawings, however, there is observed anirregular line (called “jaggy”) at an inclined edge portion when aresolution is around 600 dpi, and this is considered to be a cause ofimage quality degradation.

The most apparent method to improve the jaggy is to increase theresolution from 600 dpi to a higher resolution of 1200 dpi or the like.A problem of this method is that the number of pixels to be processedbecomes enormous (4 times larger than in the case of 600 dpi) and alonger processing time or an additional processing resource becomesnecessary.

For this problem, there has been proposed a technique to retain highresolution data adaptively and locally. In Japanese Patent Laid-Open No.2004-320361, the jaggies at edge portions are improved and high speedprocessing is performed by retaining the edge portions of an object tobe drawn in a higher resolution when a vector image thereof israsterized.

There have been some other proposals to improve the jaggy at low cost.Representative techniques thereof include an anti-aliasing processing.This is a kind of processing performed in rasterizing vector data anddetails thereof are shown in FIGS. 1A to 1E.

A pixel (FIG. 1A) is divided into fine sub-pixels (nine sub-pixels inFIG. 1B), and vector data is superimposed on the sub-pixels (FIG. 1C).An output pixel value of each of the sub-pixels is determined to be “1”when the vector data overlaps more than a half of the pixel (a region ofa drawing occupies more than a half of a sub-pixel area) and otherwisedetermined to be “0” (FIG. 1D). In this case, apparently from FIG. 1D,there are four sub-pixels of “1” in the nine sub-pixels and the outputpixel value becomes “ 4/9” (FIG. 1E).

Many improvement techniques are proposed for the anti-aliasingprocessing as described above.

In Japanese Patent Laid-Open No. 9-18710, an identifier is provided to apixel for indicating whether the pixel is rasterized from vector data atthe same time when the anti-aliasing processing is performed and alsothe direction of an edge thereof is stored. Also, the number of bits inobtained halftone data is reduced and the edge direction information isadded thereto, thereby reducing a storage area. Here, in the ninesub-pixels, a bitmap pattern and an edge direction are preliminarilyassociated with each other for each sub-pixel state (sub-pixel is “1” or“0”). With reference to the relationship associated in this manner, theedge direction is determined from the relationship obtained in FIG. 1D.In FIGS. 1A to 1E, the edge direction is “downward”.

In the Japanese Patent Laid-Open No. 2004-320361, however, one pixelvalue has multiple sets of color information, and therefore the methodis effective only in a specific color and provides restrictions to otherimage processing systems like color processing. Also the method iseffective only in rendering vector information but can not processraster images.

Also, in the Japanese Patent Laid-Open No. 9-18710, the directioninformation of an edge portion needs to be acquired from the vectorinformation and can not be acquired from the raster image. Further, themethod has a potential problem of image blurring in an image havingcomplicated edges.

As described above, the increase of resolution is effective to improvethe jaggy or the like, but invites an increase of the processing time orthe processing resource along with this increase of resolution.Accordingly, even when the resolution is increased for the improvementof the jaggy or the like, it is desirable to suppress the increase ofthe processing time or the processing resource.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image processingapparatus, image processing method, computer program, and storagemedium, which can reproduce an edge portion with a high resolution in ahigh resolution raster image, and suppress image degradation like thejaggy while suppressing processing cost thereof.

An image processing apparatus according to a first aspect of the presentinvention includes: a dividing means for dividing image data into blocksof M×N pixels (both M and N are integers of 1 or more and also at leastone of M and N is an integer of 2 or more); an acquiring means, when thenumber of colors existing in the block is 2, for acquiring shapeinformation regarding arrangement of said two colors in the block; astoring means for storing said acquired shape information; and a storingmeans for storing information regarding each of said two colors.

An image processing apparatus according to a second aspect of thepresent invention includes: a dividing means for dividing image datainto blocks of M×N pixels (both M and N are integers of 1 or more andalso at least one of M and N is an integer of 2 or more); an acquiringmeans, when the number of colors existing in the block is X (X is aninteger of M×N−1 or less), for acquiring shape information regardingarrangement of said X colors in the block; a storing means for storingsaid acquired shape information; and a storing means for storinginformation regarding each of said X colors.

An image processing method according to a third aspect of the presentinvention includes: a dividing step of dividing image data into blocksof M×N pixels (both M and N are integers of 1 or more and also at leastone of M and N is an integer of 2 or more); and a determining step ofdetermining whether or not the number of colors existing in said dividedblock is 2; when the number of colors existing in the block isdetermined to be 2 from the result of said determination, a firstacquiring step of acquiring first shape information regardingarrangement of said two colors in the block; and a storing step ofstoring said acquired first shape information and said informationregarding each of said two colors; when the number of colors existing inthe block is determined to be other than 2 from the result of saiddetermination, a representative color determining step of determining arepresentative color of the block; a second acquiring step of acquiringsecond shape information regarding arrangement of said representativecolor in the block having said representative color; and a storing stepof storing each of said acquired second shape information andinformation regarding said representative color.

An image processing method according to a fourth aspect of the presentinvention includes: a dividing step of dividing image data into blocksof M×N pixels (both M and N are integers of 1 or more and also at leastone of M and N is an integer of 2 or more); a color reducing step ofreducing the number of colors existing in the block to 2 or less foreach of said blocks; an acquiring step of acquiring shape informationregarding arrangement of said two or less colors in the block; and astoring step of storing said acquired shape information and colorinformation regarding each of said two or less colors.

An image processing method according to a fifth aspect of the presentinvention includes: a dividing step of dividing image data into blocksof M×N pixels (both M and N are integers of 1 or more and also at leastone of M and N is an integer of 2 or more); a color reducing step ofreducing the number of colors existing in the block to 2 or less foreach of said blocks; and a specific color determining step ofdetermining whether or not either one of said two or less colors is apredetermined color; when either one of said two colors is saidpredetermined color from the result of said determination, a firstacquiring step of acquiring shape information said shape informationregarding arrangement of said two colors in the block; a secondacquiring step of acquiring information regarding a color other thansaid predetermined color of said two colors; and a storing step ofstoring said acquired shape information and said information regardingthe color other than said predetermined color; when neither of said twocolors is said predetermined color from the result of saiddetermination, a representative color determining step of determining arepresentative color of the block; and a storing step of storinginformation regarding said representative color.

According to the present invention, it is possible to obtain a highquality image by suppressing image degradation such as the jaggy, whilereducing the number of pixels to be processed by sampling adaptivelyhigh resolution raster image data thereof.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are diagrams for illustrating a conventionalanti-aliasing processing;

FIG. 2 is a schematic block diagram illustrating a configuration of animage forming apparatus according to an embodiment of the presentinvention;

FIG. 3 is a diagram illustrating an outline of the image formingapparatus according to the embodiment of the present invention;

FIG. 4 is a flowchart of sub-sampling processing according to theembodiment of the present invention;

FIG. 5 is a flowchart of image processing according to the embodiment ofthe present invention;

FIG. 6 is a flowchart of the sub-sampling processing according to anembodiment of the present invention;

FIG. 7 is a flowchart showing color binarization processing according tothe embodiment of the present invention;

FIG. 8 is a diagram for illustrating the color binarization processingaccording to the embodiment of the present invention;

FIG. 9 is a flowchart of the sub-sampling processing according to theembodiment of the present invention; and

FIGS. 10A and 10B are diagrams for illustrating arrangement of twocolors in a block of 2×2 pixels according to the embodiment of thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Note that, in the drawings to bedescribed hereinbelow, components with the same function are denoted bythe same reference numerals symbols and repeated description thereofwill be omitted.

Hereinafter, an image forming apparatus according to an embodiment ofthe present invention assumes a digital composite machine but is alsoapplicable to other printing devices such as a color copier, a colorprinter, etc.

First Embodiment

(Configuration of an Image Forming Apparatus)

FIG. 2 is a schematic block diagram illustrating an outline of an imageforming apparatus according to the present embodiment.

As shown in FIG. 2, the image forming apparatus according to the presentembodiment includes an image reading part 101, an image processing part102, a storing part 103, a CPU 104, and an image output part 105. Here,the image forming apparatus can be connected to a server controllingimage data, a personal computer (PC) capable of instructing printexecution or providing the image data, and the like, via a network.

The image reading part 101 reads an image of a document and outputsimage data thereof. This image reading part 101 is a scanner, forexample.

The image processing part 102 converts printing information includingimage data input from the image reading part 101 or an externalapparatus such as a PC into intermediate information (hereinafter called“object”) and stores the object in an object buffer of the storing part103, while performing image processing such as intensity correction.Further, the image processing part 102 generates bit map data based onthe buffered object and stores it into a band buffer of the storing part103, while performing halftone processing such as dithering.

An adaptive sub-sampling processing which is characteristic to thepresent invention is performed on the bit map data in raster imageprocessing, which will be described below in detail. Here, thesub-sampling processing is resolution converting processing to convert ahigher resolution image into a lower resolution image.

The storing part 103 can include a ROM, RAM, hard disk (HD), etc. TheROM stores various kinds of control programs and image processingprograms executed by the CPU 104. The RAM is used for a reference areaand a work area where the CPU 104 stores data and various kinds ofinformation. Also, the RAM and HD are used for the above mentionedobject buffer, band buffer, and the like.

The image output part 105 forms a color image on a recording medium likea recording paper for output. This image output part 105 is anelectro-photographic printer, for example, as described below.

(Outline of the Apparatus)

FIG. 3 is a diagram illustrating an outline of the image formingapparatus according to the present embodiment.

In FIG. 3, at the image reading part 101, a document 204, an image ofwhich is to be read, is placed between a glass document plate 203 and adocument pressing board 202. When the document 204 is illuminated bylight from a lamp 205, reflected light from the document 204 is guidedby mirrors 206 and 207 and is focused on a three-line sensor 210 to forman image by a lens 208. Here, the lens 208 is provided with an infra-redcut filter 231. A mirror unit including the mirror 206 and the lamp 205is moved at a speed of V and another mirror unit including the mirror207 is moved at a speed of V/2, toward the direction indicated by thearrow by a motor (not shown in the drawing). That is, the whole surfaceof the document 204 is scanned by the movement of the mirror units in adirection (sub-scanning direction) vertical to the electrical scanningdirection (main scanning direction) of the three-line sensor 210.

The three-line sensor 210 constituted by three lines of CCDs includes aCCD 210-1 receiving red R, a CCD 210-2 receiving green G, and a CCD210-3 receiving blue B. The three-line sensor 210, by such aconfiguration, decomposes input light information into colors and readsrespective color components; red R, green G, and blue B, of the fullcolor information. Here, each of the CCDs 210-1 to 210-3 constitutingthe three-line sensor 210 has light receiving elements corresponding to8,000 pixels and can read a document having an A3 size, which is themaximum document size placeable on the glass document plate 203, with aresolution of 600 dpi in a short side direction thereof (297 mm).

A reference white board 211 is used for correcting data read by each ofthe CCDs 210-1 to 210-3 of the three-line sensor 210. The referencewhite board 211 shows a white color having approximately uniformreflection characteristics over a visible light range.

The image processing part 102 performs electrical processing on an imagesignal input from the three-line sensor 210 to generate each of thecolor component signals of magenta M, cyan C, yellow Y, and black K andsends the generated MCYK color component signals to the image outputpart 105.

In the image output part 105, the image signal(s), M, C, Y, and/or K,sent from the image reading part 101, are (is) sent to a laser driver212. The laser driver 212 drives a semiconductor laser element 213 formodulation according to the input image signal. A laser beam output fromthe semiconductor laser element 213 scans a photo-sensitive drum 217 viaa polygon mirror 214, an f-θ lens 215 and a mirror 216, and forms anelectrostatic latent image on the photo-sensitive drum 217.

There are four developing units; magenta developing unit 219, cyandeveloping unit 220, yellow developing unit 221, and black developingunit 222. Each of the four developing units contacts with thephoto-sensitive drum 217 alternately and develops the electrostaticlatent image formed on the photo-sensitive drum 217 using a toner ofcorresponding color to form a toner image. A recording paper fed from arecording paper cassette 225 is wound around a transfer drum 223 and thetoner image on the photo-sensitive drum 217 is transferred onto therecording paper.

The recording paper, onto which the toner images of four colors M, C, Y,and K are sequentially transferred in this manner, passes through afixing unit 226, and thereby toner images are fixed and then dischargedoutside the apparatus.

Next, the adaptive sub-sampling processing will be described in detail.Here, this processing is performed on the bitmap data input from theimage reading part 101 or from outside and performed in the imageprocessing part 102.

FIG. 4 shows a flowchart of the sub-sampling processing according to thepresent embodiment.

First, when high resolution image data is input from the image readingpart 101, the PC, or the like, the image processing part 102 divides theinput high resolution image data into blocks of M×N pixels (S401). Here,both M and N are integers of 1 or more and also at least one of M and Nis an integer of 2 or more. There is no reason to exclude M=N.

Here, for simplicity, a case of the block of 2×2 pixels will bedescribed. That is, input image data is divided into the blocks of 2×2pixels. For example, an input high resolution image data having1,000×1,000 pixels is divided into the blocks of 2×2 pixels, resultingin 500×500 blocks.

Next, the image processing part 102 performs color rounding processingto make similar colors to be the same color for colors of four pixels inthe one block of the image data divided in S401 (S402). This processingaverages pixels, which have levels closer to one another than to acertain threshold level, and rounds pixel values of the pixels into thesame level (color). That is, the color rounding processing may be anyprocessing if colors similar to one another are converted into the samecolor and the colors are approximated (color approximation), when aplurality of colors is included in the block. The image processing part102 acquires colors to be used in a below-described processing fromcolors included in the target block in this manner, and stores theacquired colors into the storing part 103.

Then, the image processing part 102 counts the number of colors (numberof color calculation) within the block subjected to the color rounding(S403). That is, the image processing part 102 calculates the number ofcolors existing in the target block. Here, the same color may includecolors different from each other by several levels, since the colorrounding processing has been performed as a preprocessing.

Next, attention is focused on a case the number of colors is two. Thatis, the image processing part 102 determines whether or not the numberof colors is two in the block subjected to the color rounding processing(S404). If the number of colors is determined to be two, the processproceeds to S408, and if the number of colors is determined to be nottwo, the process proceeds to S405.

When the block of 2×2 pixels has two colors, seven arrangements thereofare possible as shown in FIGS. 10A-1 to 10A-7. Even including a case thewhole block has one color, up to eight arrangements are possible. Thatis, an arrangement of two colors in the block of 2×2 pixels can berepresented by three bits. FIGS. 10B-8, 10B-9, and 10B-A to 10B-F arediagrams showing arrangements when each color in the arrangements shownin FIGS. 10A-1 to 10A-7 is inverted. These arrangement patterns arepreliminarily stored in the storing part 103.

Meanwhile, the present embodiment employs the bock of 2×2 pixels and canrepresent the arrangement of two colors with three bits. Thisarrangement of two colors corresponds to a shape of an input image ineach of the blocks and information representing the arrangement isreferred to as “shape information” in the present specification. Forexample, the above shape is an edge shape in the block including an edgeof the image.

The present embodiment divides the image data into the blocks of 2×2pixels as described above and the shape information is three-bitinformation. Here, a case of dividing an image data into the blocks of2×2 pixels is an example, and the number of bits varies depending on theabove M and N. For example, for the block of 2×2 pixels, the number ofbits increases by one bit as the size of the block increases by onepixel.

When the block has been determined to be composed of two colors in S404,the image processing part 102 detects into which arrangement the shapeof the target block is classified among the seven kinds of arrangementsshown in FIG. 11A (S408). That is, the image processing part 102 detectswhich arrangement pattern corresponds to the arrangement pattern of twocolors in the target block referring to the arrangement patters storedin the storing part 103 (patterns shown in FIG. 10A). Then, the imageprocessing part 102 acquires three-bit shape information for the targetblock from the above detection result and stores the information intothe storing part 103.

After that, the image processing part 102 acquires color information foreach of the two colors and the shape information in the target block of2×2 pixels, and stores them into the storing part 103 (S409). The colorinformation is the information representing the color obtained in S402by the color rounding. The block has two colors as described above andthe image processing part 102 acquires the color information for a firstcolor and the color information for a second color.

When these first and second colors are determined in the block of 2×2pixels, a color of a predetermined pixel is determined to be the firstcolor and colors in the other pixels the same color as that of thepredetermined pixel is also determined to be the first color. In theabove block, color other than the first color may be determined to bethe second color. For example, in the block of 2×2 pixels shown in FIG.10A, the image processing part 102 acquires the color information forthe first color, by selecting an upper left pixel for the predeterminedpixel and by determining the color of the pixel and the same color inother pixels to be the first color. Also, the image processing part 102acquires the color information for the second color by determiningcolors other than the first color to be the second color in the block.In this manner, the image processing part 102 outputs the three-bitshape information of the target block and the color information for thefirst and second colors in S409.

When the number of colors in the target block is other than two in S404,the image processing part 102 calculates an average color in the targetblock (S405). In the case of the block of 2×2 pixels, the imageprocessing part 102 acquires the average color as a representative colorin the block when the number of colors in the block is 1, 3, or 4. Thisaverage color is obtained by calculating an average pixel value, forexample. That is, the image processing part 102 functions as arepresentative color determining means determining a representativecolor of the block having the number of colors other than two.

In the present specification, the “representative color” is a colorwhich is owned by each block when the present sub-sampling processinghas finished in a case the block has colors other than two colors, andis a representative color of the block acquired based on colors of theblock in input image data.

Subsequently, the image processing part 102 allots the calculatedaverage color to the first color and second color (S406). That is, theimage processing part 102 acquires the color information for the firstcolor and the color information for the second color by allotting theaverage color acquired in S405 to the first color and second color.Thereby, the shape of the block in this case is the arrangement patternshown in FIG. 10A-0.

Note that, in a case the block has three or four colors, a kind ofprocessing may be performed so as to reduce the number of colors to two,although the present embodiment employs one representative color. Inthis case, the shape of the block is any of the arrangement patternsshown in FIGS. 10A-1 to 10A-7.

Next, the image processing part 102 outputs the shape information of thetarget block (called second shape information for discriminating theshape information (first shape information) detected in S408 therefrom)and the color information for the first color and the color informationfor the second color (S407). The image processing part 102 acquires thesecond shape information in this manner, and thus functions also as asecond shape information acquiring means (also called a second acquiringmeans for discriminating the function in S408 (a first acquiring unit)therefrom). That is, the image processing part 102 detects the shape ofthe target block as in S408 to acquire the three-bit shape informationand stores the information into the storing part 103. Further, the imageprocessing part 102 stores the color information for the first color andthe color information for the second color acquired in S406 into thestoring part 103.

Subsequently, the image processing part 102 determines whether or notthe shape information and the color information for the first and secondcolors have been acquired for all the blocks of the image data dividedin S401. If it is determined that the information has been acquired forall the blocks, the present sub-sampling processing is terminated. Onthe other hand, if it is determined that the information has not beenacquired for all the blocks, the process returns to S401, and the stepsof S401 to S410 are repeated until the acquisition of the informationfinishes for all the blocks.

By this processing, the image data is sampled into two sets of colordata (color information for the first and second colors) in a portionhaving a sharp edge (large color difference) and edge data (shapeinformation). The data amount thereof is reduced to (a half of the dataamount of input image color data)+(3 bits of edge shape data) in a caseof the block of 2×2 pixels.

For example, when an input image having 1,000×1,000 pixels is processedby use of the block of 2×2 pixels, the image data is sampled into500×500 pixel data of a first color image, 500×500 pixel data of asecond color image, and 500×500 pixel data of the edge shape data (shapeinformation).

The present embodiment divides input image data into the blocks of 2×2pixels and acquires the color information for the first color and thecolor information for the second color for each of the blocks accordingto the flow of FIG. 4. The above first color image is image datacollecting the first color in each block. Since one block is a unitpixel here, a unit of 2×2 pixels of the input image data is one pixel ofthe first color image. Therefore, the first color image has 500×500pixels, when the input image data has 1,000×1,000 pixels. Similarly, thesecond color image is image data collecting the second color of eachblock.

Note that the first color and the second color are not predeterminedcolors but colors specific to each block in the present embodiment. Thatis, the first color and the second color are colors determined forallotting the two colors for the first color image and the second colorimage, respectively, when colors in the block are divided into twocolors. Therefore, when the block has originally two colors or when theblock has two colors as the result of the color rounding, the two colorsare the first color and the second color, respectively. Also, when theblock has colors other than two colors, the representative color of theblock is the first color and the second color. The number of theserepresentative colors may be one as in the present embodiment or may betwo as described hereinafter.

In up-sampling, the image processing part 102 rearranges the first andsecond colors by referring to the edge shape data (shape information)stored in the storing part 103 for each block to obtain the originalimage. That is, the image processing 102 reads the shape information,the color information for the first color, and the color information forthe second color for each block from the storing part 103. Then, theimage processing part 102 arranges the first color and the second coloralong an original shape for each block according to the read-out shapeinformation, color information for the first color, and colorinformation for the second color. Subsequently, the image processingpart 102 combines the arranged blocks to generate a final output image.Here, the up-sampling is a resolution converting processing to convertan image, the resolution of which is reduced by sub-sampling, into ahigh resolution image.

The present embodiment stores the edge shape (shape of the two colorarrangement) as well as the two colors in each block, and thereby canrestore an edge of original high resolution data itself. Blocks havingtwo colors are frequently found in a portion of a character image, aline drawing image, or the like which usually requires edge sharpness,while blocks having three or more colors are frequently found in aportion of a natural image or the like which has not particularlyrequired a high resolution.

Next, a flow of the whole image processing will be described withreference to FIG. 5.

Typically, the image processing as shown in S502 to S506 in FIG. 5 isprepared for processing an image in a printer. This image processinggroup is performed on a raster image, and therefore the number of pixelsto be processed directly affects a processing load. For example, whenthe image processing is compared between 1,200 dpi and 600 dpi, theformer case needs a four times longer processing time than the lattercase.

However, when the sub-sampling processing described above is performedat the top of the image processing steps (S501), the number of pixelsinput to the succeeding image processing steps (S509) is reduced to ahalf and the processing time becomes approximately a half. Also, theimage processing steps can perform the processing thereof withoutparticular awareness that the input image has been provided with thesub-sampling and can divert existing image processing technique.Finally, the up-sampling processing (S507) and a halftone processing(S508) are performed and thereby it becomes possible to realize a highresolution image output with little jaggy by a low cost imageprocessing.

The flow of FIG. 5 will be described below.

First, in S501, the image processing part 102 performs the sub-samplingprocessing according to the flow described in FIG. 4. That is, the imageprocessing part 102 acquires the shape information and the colorinformation for the first and second colors for all the divided blocksand also acquires the first color image, the second color image, and theshape information for each block. At this time, even if the input imagedata is high resolution data, the input image data is divided into theblocks of 2×2 pixels and the first color image and the second colorimage are acquired, and thereby the data amount thereof is reduced to ahalf for image data used in the succeeding image processing step S509.Therefore, in the image processing step S509, the processing isperformed for a resolution lower than that of the input image data.

Next, the image processing part 102 provides the image processing ofS509 on the first color image and the second color image. That is, theimage processing part 102 performs compression processing for storingthe image data into the memory or the hard disk (S502). Subsequently,the image processing part 102 reads the compressed data from the memoryor the hard disk and performs decompression development processing torestore a raster image (S503). The image data is subjected to colorconversion processing to be compatible with an output color space(S504), and then subjected to intensity adjustment (S505) and outputgamma correction (S506).

Next, the image processing part 102 restores an image for each blockaccording to the first color image and the second color image subjectedto the image processing and the shape information acquired for eachblock (S507). Then, the image processing part 102 performs halftoneprocessing on the restored image (S508) and the process moves to thenext image output operation.

The present embodiment reduces the number of pixels to be processed inthe image processing such as color conversion or the like for reducingthe jaggy even in a high resolution image, that is, the resolution isreduced. Thereby, the processing time or the processing resource isprevented from increasing even when an input image is a high resolutionimage. Accordingly, it is possible to output a high quality image, inwhich the jaggy or the like is reduced at low cost and in a shortprocessing time.

The present embodiment divides the input image into the blocks of 2×2pixels, for example, and makes the divided block as a pixel unit, and,for each block, retains the arrangement pattern (shape) of the twocolors included in the block. Since the shape in each block is retainedas described above, the original image can be restored from the imagesubjected to the image processing and sub-sampling, even though theresolution thereof has been reduced by the sub-sampling for reducing theprocessing time or the processing resource. That is, retaining the shapein each block allows even an edge portion to be restored precisely,while the processing time or the processing resource is reduced.

In the present embodiment, it is also important to make the number ofcolors in the block to two for reducing the number of input pixels inthe image data used for the image processing. For this purpose, thecolor rounding processing is performed in S402 or the representativecolor is determined in S405 and S406 of FIG. 4. In the presentembodiment, it is necessary to reduce the number of colors to less thana maximum number of colors allowable in each block of the input imagefor reducing the number of input pixels. For example, in a case theimage data is divided into the blocks of 2×2 pixels, each block has fourcolors at maximum. In this case, four sets of color information areacquired for each block and the processing time or the processingresource can not be reduced. Therefore, it becomes important to reducethe number of color information sets acquired for each block to lessthan the maximum number of colors. Accordingly, the present embodimentperforms the color rounding processing or determines the representativecolor.

The present embodiment uses a raster image of bitmap data as an inputimage and thereby can perform predetermined processing for each pixelsuch as the color rounding processing and the like as described above.Also, by using the raster image, it is possible to perform thesub-sampling and to reduce the resolution thereof only during the imageprocessing.

Note that, although the image data is divided into the blocks of 2×2pixels in all the descriptions hereinabove, the present embodiment isnot limited to the case. Also, although the two colors in the blockthemselves are determined to be the representative color of the twocolors in the above description, how to determine the representativecolor is not limited to this method. For example, it is possible toperform the sub-sampling using the average color and an upper left pixelvalue as representative colors. From these average color, upper leftpixel value, and edge shape data, the second color composing the blockis easily restored by calculation.

Also, although representation by color is used for describing a state ina block hereinabove, the present embodiment is obviously effective forimage data such as a gray scale image data which does not have colorsand is not limited to the case of RGB or CMYK.

Second Embodiment

Hereinafter, there will be described image processing according to asecond embodiment of the present invention.

The present embodiment performs color binarization processing to reducethe number of colors in a block to two instead of counting the number ofcolors in the block of M×N pixels (e.g., 2×2 pixels), in the adaptivesub-sampling processing.

FIG. 6 shows a flow of sub-sampling processing according to the presentembodiment.

First, the image processing part 102 divides an input high resolutionimage into blocks of 2×2 pixels (S601) as in S401 of FIG. 4.Subsequently, the image processing part 102 performs processing toreduce the number of pixel colors to two in each of the blocks, andconverts the pixel colors into only two kinds of colors or one kind ofcolor (two colors or less) in the block (S602). This processing will bedescribed hereinafter with reference to FIG. 7. Even when the block iscomposed of three or four colors, the present embodiment performs thecolor reduction processing and thereby can make the color arrangement inthe block to correspond to any color arrangement as shown in FIG. 10A.Accordingly, averaging of three or four colors as in the firstembodiment is not performed, and thereby a higher quality image can beoutput.

Subsequently, the image processing part 102 detects shape information ofthe block composed of two colors or one color, referring to thearrangement patterns stored in the storing part 103 as in S408 (S603).Next, the image processing part 102 acquires color information for afirst color and color information for a second color from the two colorsacquired in S602 and stores the information into the storing part 103 inS604 as in S409. Further, the image processing part 102 stores the shapeinformation acquired in S603 into the storing part 103. Here, when thecolors of the block are converted into an average color (the number ofcolors is reduced to one) in S602 as will be described hereinafter orthe target block of 2×2 pixels is originally composed of one color, theimage processing part 102 allots the one color to the first and secondcolors.

Next, the image processing part 102 determines whether or not the shapeinformation and the color information for the first and second colorshave been acquired for all the divided blocks in S605. If it isdetermined that the information has been acquired for all the blocks,the present sub-sampling processing is terminated. On the other hand, ifit is determined that the information has not been acquired for all theblocks, the process returns to S601 and the steps S601 to S605 arerepeated until the acquisition of the information finishes for all theblocks.

In this manner, a first color image, a second color image, and the shapeinformation are acquired, as in the first embodiment.

FIG. 7 shows a detailed flow of the color binarization processing, i.e.,color reduction processing for colors in the block according to thepresent embodiment.

First, the image processing part 102 detects two pixels that have thelargest inter-pixel distance in the block of 2×2 pixels composed of oneto four colors and calculates the distance (S701). In the presentspecification, “the largest inter-pixel distance” is the largestdistance between coordinates of pixels in the block in the color space.FIG. 8 shows an example of this color space.

In FIG. 8, each axis represents gradation values (0 to 255) of red (R),gradation values (0 to 255) of green (G), or gradation values (0 to 255)of blue (B). Reference numeral 81 indicates the block of 2×2 pixels,reference numerals 811 to 814 indicate pixels belonging to the block 81.Also, reference numeral 83 indicates the color coordinates of the pixel811, reference numeral 84 indicates the color coordinates of the pixel812, reference numeral 85 indicates the color coordinates of the pixel813, and reference numeral 86 indicates the color coordinates of thepixel 814. In FIG. 8, the distance between the coordinates 83 and thecoordinates 86 is the largest and the distance thereof is the largestinter-pixel distance.

Also, the distance Dist is calculated by the following formula, forexample.Dist=|RA−RD|+|GA−GD|+|BA−BD|  [Formula 1]

where (RA, GA, BA) is a pixel value at point A, and (RD, GD, BD) is apixel value at point B.

That is, in S701, colors of respective pixels in the target block areplotted in the color space shown in FIG. 8, distances betweencoordinates thereof are calculated, and the largest inter-pixel distanceis obtained by using the above formula.

Next, the image processing part 102 determines whether or not thelargest inter-pixel distance calculated in S701 is within a thresholdvalue (S702). If it is determined that the largest inter-pixel distanceis within the threshold value, the process proceeds to S707, and if itis determined that the largest inter-pixel distance is larger than thethreshold value, the process proceeds to S703.

Here, when the block has originally only one color, all the colorcoordinates exist on the same point and the largest inter-pixel distancebecomes zero. In this case, the process proceeds to S707.

When it is determined that the largest inter-pixel distance is withinthe threshold value, the image processing part 102 determines that allthe colors in the target block are the same. That is, the imageprocessing part 102 calculates an average color in the block of 2×2pixels (S707), replaces all of the four pixel colors by the averagecolor, and stores the average color as the first color information andthe second color information into the storing part 103 (S708).

On the other hand, when the largest inter-pixel distance is larger thanthe threshold value, the image processing part 102 determines that aplurality of colors exists in the block and performs the colorbinarization. That is, the image processing part 102 determines toselect the pixel values of two pixels having the largest inter-pixeldistance for initial colors (S703). In the present specification, the“initial color” is a reference color for the color binarization. Thatis, the colors of the two pixels having the largest inter-pixel distancebecome the initial colors and it is determined to which of the initialcolors is closer each of the remaining two pixel colors, as describedbelow. In FIG. 8, the distance between the coordinates 83 and thecoordinates 86 is the largest inter-pixel distance and the colors of thepixel 811 and pixel 814 become the initial colors.

Next, the image processing part 102 calculates to which of the initialcolors determined in S703 is closer each of the remaining two pixelcolors and groups each of the remaining two pixel colors together withone of the initial colors closer thereto (S704). In FIG. 8, the imageprocessing part 102 calculates distances between each of the coordinates84 and 85 and each of the coordinates 83 and 86, and determines to whichof the coordinates 83 and 86 is closer each of the coordinates 84 and85. Since the coordinates 84 is closer to the coordinates 83, the pixel812 is grouped together with the pixel 811. Also, since the coordinates85 is closer to the coordinates 86, the pixel 813 is grouped togetherwith the pixel 814.

Subsequently, the image processing part 102 calculates therepresentative color of the grouped colors (average color) (S705). Thatis, the image processing part 102 calculates an average color(representative color) of the color of the pixel 811 determined as theinitial color and the color of the pixel 812 grouped together with thepixel 811. Similarly, the image processing part 102 calculates anotheraverage color (representative color) of the color of the pixel 814determined as the initial color and the color of the pixel 813 groupedtogether with the pixel 814. These two average colors become therepresentative colors of the groups, respectively.

Next, the image processing part 102 replaces the pixel values in theblock by the representative colors, and makes the representative colorsto be the first color and the second color. Then the image processingpart 102 acquires the color information for the first color and thecolor information for the second color to store the information into thestoring part 103 (S706). This series of processing replaces colors ofall the blocks of 2×2 pixels by one kind of color or two kinds ofcolors.

Meanwhile, edge shapes (color arrangements) in a case of the 2×2 pixelblock are eight kinds shown in FIG. 11A as in the first embodiment.Therefore, the shape information can be represented by three bits andthe data amount thereof can be reduced. The present embodiment providesany edge shape also to the block composed of three or four colorswithout fail and an edge amount to be output is increased compared withthe first embodiment. Therefore, while image quality can be improvedcompared with an image up-sampled in the first embodiment, a data sizebecomes larger in such a case that the shape information is compressedto be transmitted.

Hereinabove, an example of the color reduction processing has beendescribed in detail, but the color reduction processing is not limitedto this example. Also, the color reduction processing is performed onthe basis of the distance in the RGB space in FIG. 8, but the colorreduction process is not limited to this case.

Third Embodiment

Hereinafter, image processing according to a third embodiment of thepresent invention will be described.

In the adaptive sub-sampling processing, the third embodiment performsthe color binarization processing in a block of M×N pixels (e.g., 2×2pixels), and then determines whether either of colors in the block is aspecific color (e.g., white) and retains an edge shape (shapeinformation) in the case of the specific color.

FIG. 9 shows a flow of the sub-sampling processing according to thepresent embodiment.

In FIG. 9, S901 and S902 are the same as S601 and S602 shown in FIG. 6,respectively, and description thereof will be omitted.

After reducing the number of colors in the block to two in S902, theimage processing part 102 determines whether or not either of the tworeduced colors is a specific color (S903) That is, the image processingpart 102 functions as a specific color determining means determiningwhether or not either of two colors is a predetermined color.Specifically, the specific color may be preliminarily stored in thestoring part 103, and the image processing part 102 may perform theabove determination from information about the two reduced colorsreferring to the specific color stored in the storing part 103. Thespecific color may be preliminarily stored in the storing part 103 ormay be set according to information input by a user via the input partor the PC provided in the image forming apparatus. In the presentspecification, the “specific color” is a color preliminarily determinedbefore the present sub-sampling processing.

Here, when the block has originally only one color and the color is thespecific color, the process proceeds to S904.

Next, when one of the two colors is determined to be the specific colorin S903, the image processing part 102 detects the shape information ofthe block composed of the two colors referring to the arrangementpatterns stored in the storing part 103 as in S408 (S904). Here, inaddition to the shape information shown in FIG. 10A, the presentembodiment needs to retain the shape information of the arrangementsshown in FIG. 10B which represents inverted colors of the arrangementsshown in FIGS. 10A-1 to 10A-7. Therefore, the shape information has fourbits in the present embodiment.

Subsequently, the image processing part 102 outputs the shapeinformation acquired in S903 and information about a color which is notthe specific color (S905). That is, the image processing part 102 storesthe shape information into the storing part 103 and further, making thecolor other than the specific color in the two reduced colors as anoutput color of the target block, acquires the color information aboutthe output color and stores the color information into the storing part103.

Here, in the present specification, the “output color” is a color to beprocessed in the image processing (e.g., S509 shown in FIG. 5) after thepresent sub-sampling processing.

Note that, in a case the block has originally only one color and thecolor is the specific color, the color is determined to be the outputcolor.

On the other hand, when both of the two colors are determined not to bethe specific color, the image processing part 102 calculates an averagecolor in the block for the representative color of the target block of2×2 pixels (S906). Subsequently, the image processing part 102 outputsthe representative color (average color) calculated in S906 (S907). Thatis, the image processing part 102 acquires the color information for theoutput color, by making the average color calculated in S907 to be theoutput color of the target block, and stores the information into thestoring part 103. Note that the shape information is not acquired inthis case.

Next, the image processing part 102 determines whether or not the aboveacquisition has finished for all the blocks of the image data divided inS908. If it is determined that the acquisition has finished for all theblocks, the present sub-sampling processing is terminated. On the otherhand, if it is determined that the acquisition has not finished for allthe blocks, the process returns to S901 and the steps S901 to S908 arerepeated until the acquisition finishes for all the blocks.

In the present embodiment, when an input image with 1,000×1,000 pixelsis input, the number of pixel to be output becomes 500×500 pixels, aquarter of the number of input pixels, by the sampling. This number iseven a half of that in the first embodiment or the second embodiment. Inthis case, the edge shape (shape information) is retained only for theblock composed of the specific color.

By defining a color to be retained at least for an edge as the specificcolor, the number of pixels to be processed can be reduced. For,example, when the specific color is defined to be white, an edge shapeof a pixel neighboring a white background is retained and an edge shapeof a document drawn on a white background is preserved. Also, white is acolor which is usually not affected by color processing (colorconversion, intensity adjustment, gamma correction, etc.) after thepresent processing, differently from other colors, and can be frequentlypassed through the processing without a problem. When the specific coloris defined to be another color, change of the color by the followingcolor processing may be pursued.

Also, hereinabove, an example has been described in a case using onespecific color, but another case may be applied. That is, by making apair of a specific color and an index thereof for a plurality ofspecific colors, a combination of a certain specific color (index) andthe second color may be retained for each block. For example, fourcolors, white (0), red (1), blue (2), and black (3) (two-bit indexes),are used for the specific colors and one of the colors may be retainedtogether with the second color acquired hereinabove.

Fourth Embodiment

In the first embodiment, in a case the block has two colors, the colorsand the shape information thereof are acquired, and, in a case the blockhas not two colors, the representative color and the shape informationthereof are acquired. However, the embodiment is not limited to thiscase. As described hereinabove, an important point in an embodiment ofthe present invention is to reduce the number of colors in the dividedblock to smaller than maximum number of colors allowable in each blockof the input image in the sub-sampling. That is, the number of colorinformation sets acquired by the sub-sampling is made smaller than themaximum number of colors for each block. This maximum number of colorsis M×N in a case the image is divided into blocks of M×N pixels in theimage dividing processing. Here, if the number of colors is reduced tosmaller than (M×N−1) for each block, the processing time or theprocessing cost can be reduced.

Accordingly, in the present embodiment, the image processing part 102determines whether or not the block of M×N pixels has X colors (X is aninteger of (M×N−1) or less), and, in the case of X colors, acquires therespective color information and the shape information of the block. Inthe case the block has not X colors, the image processing part 102calculates an average color in the block and determines therepresentative color (one color) or acquires the representative color ofthe X colors form colors other than the X colors in the block. In thiscase, when the block has a larger number of colors (Y colors) than thatof the X colors, the Y colors may be reduced to the X colors by thecolor reduction processing. Also, when the block has a smaller number ofcolors (Z colors) than that of the X colors, some colors duplicated fromthe Z colors may be allotted to some of the representative colors suchthat the X representative colors are acquired. This acquired informationis stored in the storing part 103 as in the first embodiment.

In the second and third embodiments, when a block has more than twocolors, the colors are reduced to two colors, but an embodiment of thepresent invention is not limited to this case. That is, in the presentembodiment, the image processing part 102 reduces the number of colorsin the block of M×N pixels to X.

After that, the color information for each of the X colors and the shapeinformation of the block may be acquired as in the second embodiment.Alternatively, the specific color may be used as in the thirdembodiment.

Other Embodiments of the Present Invention

The scope of the foregoing embodiments includes a processing methodwhich stores a program in a storage medium for operating theconfigurations of the foregoing embodiments, reads the program stored inthe storage medium as codes, and causes a computer to execute the codesso as to realize the functions of the foregoing embodiments. Also, thescope of the foregoing embodiments includes not only the storage mediumstoring the above computer program but also the computer program itself.

Such storage media includes a floppy disk (registered trademark), disk,hard disk, optical disk, magneto-optical disk, CD-ROM, magnetic tape,non-volatile memory card, ROM, etc.

Also, not limited to a system executing the processing using only aprogram stored in the above mentioned storage medium, a system thatoperates on an OS, in conjunction with other software and functions ofan extension board, to execute the operations of the foregoingembodiments, falls within the scope of the foregoing embodiments.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2007-109694, filed Apr. 18, 2007, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image processing apparatus, comprising: adividing unit configured to divide image data into blocks of M×N pixels(both M and N being integers that are 1 or more and also at least one ofM and N being an integer that is 2 or more); a number of colorscalculating unit configured to calculate the number of colors withineach of the blocks; an acquiring unit configure to acquire, in a casewhere the number of colors existing in a block calculated by said numberof colors calculating unit is 2, shape information regarding arrangementof the two colors in the block; and a storing unit configured to storethe acquired shape information and information regarding each of the twocolors.
 2. The image processing apparatus according to claim 1, furthercomprising: a number of colors determining unit configured to determinewhether or not the number of colors is 2; a representative colordetermining unit configured to determine, in a case where the number ofcolors is determined to be other than 2 by said number of colorsdetermining unit, a representative color of the block having the numberof colors other than 2; a second acquiring unit configured to acquiresecond shape information regarding arrangement of the representativecolor in the block having the representative color; and a second storingunit configured to store the acquired second shape information and thedetermined representative color, wherein said acquiring unit acquiresthe shape information in a case where said number of colors determiningunit determines the number of colors to be
 2. 3. The image processingapparatus according to claim 2, wherein said number of colorscalculating unit includes a color approximating unit configured toconvert similar colors into the same color for a color of each pixel inthe block.
 4. The image processing apparatus according to claim 2,wherein said representative color determining unit acquires an averagecolor of the colors in the block and makes the average color therepresentative color.
 5. The image processing apparatus according toclaim 1, further comprising: a color reducing unit configured to reducethe number of colors existing in the block to 2 or less for each of theblocks; a second acquiring unit configured to acquire, in a case wherethe number of colors is 1, second shape information regardingarrangement of the one color in the block; and a second storing unitconfigured to store the acquired second shape information andinformation regarding the one color.
 6. The image processing apparatusaccording to claim 1, further comprising: a specific color determiningunit configured to determine whether or not either one of the two colorsis a predetermined color; and second acquiring unit configured toacquire information regarding a color other than the predetermined colorof the two colors, wherein said storing unit configured to storeinformation regarding each of the two colors does not store informationregarding the predetermined color.
 7. The image processing apparatusaccording to claim 1, further comprising: a specific color determiningunit configured to determine whether or not either one of the two colorsis a predetermined color; and a second acquiring unit configured toacquire information regarding a color other than the predetermined colorof the two colors, wherein said storing unit configured to storeinformation regarding each of the two colors stores an index ofinformation regarding the predetermined color.
 8. The image processingapparatus according to claim 6, further comprising: a representativecolor determining unit configured to determine a representative color ofthe block, in a case where said specific color determining unitdetermines neither of the two colors to be the predetermined color; anda second storing unit configured to store the determined representativecolor, wherein said acquiring unit does not acquire the shapeinformation in a case where said specific color determining unitdetermines neither of the two colors to be the predetermined color. 9.The image processing apparatus according to claim 6, further comprisinga color reducing unit configured to reduce the number of colors existingin the block to 2 or less for each of the blocks.
 10. The imageprocessing apparatus according to claim 1, further comprising: anarranging unit configured to arrange the two colors along thearrangement on the basis of the shape information and the informationregarding each of the two colors for each of the blocks; and agenerating unit configured to generate an output image by combining theblocks where the arrangement has finished.
 11. The image processingapparatus according to claim 1, wherein the image data is restored foreach of the blocks on the basis of the shape information and theinformation regarding each of the two colors, stored in said storingunit.
 12. A non-transitory, computer-readable storage medium storing, inexecutable form, a computer-readable program for causing a computer tofunction as an image processing apparatus according to claim
 1. 13. Theimage processing apparatus according to claim 1, further comprising: arestoring unit configured to restore, for each block, the image databased on shape information and information regarding each of the twocolors stored in said storing unit.
 14. An image processing apparatus,comprising: a dividing unit configured to divide image data into blocksof M x N pixels (both M and N being integers that are 1 or more and alsoat least one of M and N being an integer that is 2 or more); a number ofcolors calculating unit configured to calculate the number of colorswithin each of the blocks; an acquiring unit configured to acquire, in acase where the number of colors existing in a block calculated by saidnumber of colors calculating unit is X (X being an integer that is M×N−1or less), shape information regarding arrangement of the X colors in theblock; and a storing unit configured to store the acquired shapeinformation and information regarding each of the X colors.
 15. An imageprocessing method, comprising: a dividing step of dividing image datainto blocks of M×N pixels (both M and N being integers that are 1 ormore and also at least one of M and N being an integer that is 2 ormore); and a determining step of determining whether or not the numberof colors existing in the divided block is 2; performing, when thenumber of colors existing in the block is determined to be 2 from theresult of said determination, a first acquiring step of acquiring firstshape information regarding arrangement of the two colors in the block;and a storing step of storing the acquired first shape information andthe information regarding each of the two colors; and performing, whenthe number of colors existing in the block is determined to be otherthan 2 from the result of said determination, a representative colordetermining step of determining a representative color of the block; asecond acquiring step of acquiring second shape information regardingarrangement of the representative color in the block having therepresentative color; and a storing step of storing each of the acquiredsecond shape information and information regarding the representativecolor, wherein each of said steps is performed using a computer.
 16. Animage processing method, comprising: a dividing step of dividing imagedata into blocks of M×N pixels (both M and N being integers that are 1or more and also at least one of M and N being an integer that is 2 ormore); a color reducing step of reducing the number of colors existingin the block to 2 or less for each of the blocks; an acquiring step ofacquiring shape information regarding arrangement of the two or lesscolors in the block; and a storing step of storing the acquired shapeinformation and color information regarding each of the two or lesscolors, wherein each of said steps is performed using a computer.
 17. Animage processing method, comprising: a dividing step of dividing imagedata into blocks of M×N pixels (both M and N being integers that are 1or more and also at least one of M and N being an integer that is 2 ormore); a color reducing step of reducing the number of colors existingin the block to 2 or less for each of the blocks; and a specific colordetermining step of determining whether or not either one of the two orless colors is a predetermined color; performing, when either one of thetwo colors is the predetermined color from the result of saiddetermination, a first acquiring step of acquiring shape information,the shape information regarding arrangement of the two colors in theblock; a second acquiring step of acquiring information regarding acolor other than the predetermined color of the two colors; and astoring step of storing the acquired shape information and theinformation regarding the color other than said the predetermined color;and performing, when neither of the two colors is the predeterminedcolor from the result of said determination, a representative colordetermining step of determining a representative color of the block; anda storing step of storing information regarding the representativecolor, wherein each of said steps is performed using a computer.
 18. Animage processing method, comprising the steps of: dividing image datainto blocks of M×N pixels (both M and N being integers that are 1 ormore and also at least one of M and N being an integer that is 2 ormore); calculating the number of colors within the block for each block;acquiring, in a case where the number of colors calculated in saidcalculating step existing in the block is two, shape informationregarding arrangement of two colors in the block; and storing theacquired shape information and information regarding each of the twocolors, wherein each of said steps is performed using a computer.
 19. Animage processing method, comprising the steps of: dividing image datainto blocks of M×N pixels (both M and N being integers that are 1 ormore and also at least one of M and N being an integer that is 2 ormore); calculating the number of colors within the block for each block;acquiring, in a case where the number of colors calculated in saidcalculating step existing in the block is X (X being an integer of M×N−1or less), shape information regarding arrangement of the X colors in theblock; and storing the acquired shape information and informationregarding each of the X colors, wherein each of said steps is performedusing a computer.
 20. An image processing apparatus, comprising: adividing unit configured to divide image data into blocks of M×N pixels(both M and N being integers that are 1 or more and also at least one ofM and N being an integer that is 2 or more); a color reducing unitconfigured to reduce the number of colors existing in the block to twoor less for each of the blocks; an acquiring unit configured to acquireshape information regarding arrangement of the two or less colors in theblock; and a storing unit configured to store the acquired shapeinformation and color information regarding each of the two or lesscolors.